ABSTRACT Postpartum weight retention (i.e., not losing weight gained in pregnancy) affects 60% of women one year after delivery, and many gain additional weight soon thereafter. Postpartum weight retention is associated with the metabolic syndrome and obesity, which in turn increase risks for cardiovascular disease, diabetes, and adverse perinatal outcomes in later pregnancies. To date, lifestyle interventions to reduce postpartum weight retention have met with limited success. To inform new interventions, this observational study will fill gaps in knowledge about what motivates women to engage in healthy lifestyle behaviors during the unique transition from pregnancy, when many women strive to make lifestyle changes to support a healthy delivery, to the postpartum period, characterized by the intensified demands of parenthood such as sleep disruption, breastfeeding, caretaking, and workforce transitions. Our organizing hypothesis is that enjoyment of specific lifestyle behaviors (physical activity, self-weighing, and healthy eating), mediated by self-efficacy, is a distal motivational determinant that longitudinally predicts corresponding lifestyle behaviors, postpartum weight retention, and metabolic syndrome, while accounting for relevant factors unique to this stage of life (e.g., sleep disruption). To achieve our goal of identifying modifiable targets for future interventions, we will conduct a longitudinal study that efficiently leverages an ongoing pregnancy cohort of racially/ethnically diverse women (R01 ES019196). This proposal will expand follow-up for 550 women with assessments of enjoyment, self- efficacy, behavior, weight, and other factors at 6 weeks and 6, 12, 18, and 24 months postpartum. Metabolic syndrome will be assessed in pregnancy using previously collected blood samples, and at 6, 12, and 24 months postpartum using newly collected samples. Strengths include a diverse sample and theory-driven, longitudinal repeated measures design. We expect the findings to advance our understanding of modifiable, individual-level determinants of obesity and adverse cardiometabolic outcomes in diverse populations, using a mechanistic approach to behavior change over time.